Nicotinic receptor channelopathies and epilepsy

New Multitarget Molecules Derived from Caffeine as Potentiators of the Cholinergic System

Cholinergic deficit is a characteristic factor of several pathologies, such as myasthenia gravis, some types of congenital myasthenic syndromes, and Alzheimer's Disease. Two molecular targets for its treatment are acetylcholinesterase (AChE) and nicotinic acetylcholine receptor (nAChR). In previous studies, we found that caffeine behaves as a partial nAChR agonist and confirmed that it inhibits AChE. Here, we present new bifunctional caffeine derivatives consisting of a theophylline ring connected to amino groups by different linkers. All of them were more potent AChE inhibitors than caffeine. Furthermore, although some of them also activated muscle nAChR as partial agonists, not all of them stabilized nAChR in its desensitized conformation. To understand the molecular mechanism underlying these results, we performed docking studies on AChE and nAChR. The nAChR agonist behavior of the compounds depends on their accessory group, whereas their ability to stabilize the receptor in a desensitized state depends on the interactions of the linker at the binding site. Our results show that the new compounds can inhibit AChE and activate nAChR with greater potency than caffeine and provide further information on the modulation mechanisms of pharmacological targets for the design of novel therapeutic interventions in cholinergic deficit.

Nicotinic acetylcholine receptors in neurological and psychiatric diseases

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that are widely distributed both pre- and post-synaptically in the mammalian brain. By modulating cation flux across cell membranes, neuronal nAChRs regulate neuronal excitability and the release of a variety of neurotransmitters to influence multiple physiologic and behavioral processes including synaptic plasticity, motor function, attention, learning and memory. Abnormalities of neuronal nAChRs have been implicated in the pathophysiology of neurologic disorders including Alzheimer's disease, Parkinson's disease, epilepsy, and Tourette´s syndrome, as well as psychiatric disorders including schizophrenia, depression, and anxiety. The potential role of nAChRs in a particular illness may be indicated by alterations in the expression of nAChRs in relevant brain regions, genetic variability in the genes encoding for nAChR subunit proteins, and/or clinical or preclinical observations where specific ligands showed a therapeutic effect. Over the past 25 years, extensive preclinical and some early clinical evidence suggested that ligands at nAChRs might have therapeutic potential for neurologic and psychiatric disorders. However, to date the only approved indications for nAChR ligands are smoking cessation and the treatment of dry eye disease. It has been argued that progress in nAChR drug discovery has been limited by translational gaps between the preclinical models and the human disease as well as unresolved questions regarding the pharmacological goal (i.e., agonism, antagonism or receptor desensitization) depending on the disease.

The importance of ligand gated ion channels in sleep and sleep disorders

On average, humans spend about 26 years of their life sleeping. Increased sleep duration and quality has been linked to reduced disease risk; however, the cellular and molecular underpinnings of sleep remain open questions. It has been known for some time that pharmacological modulation of neurotransmission in the brain can promote either sleep or wakefulness thereby providing some clues about the molecular mechanisms at play. However, the field of sleep research has developed an increasingly detailed understanding of the requisite neuronal circuitry and key neurotransmitter receptor subtypes, suggesting that it may be possible to identify next generation pharmacological interventions to treat sleep disorders within this same space. The aim of this work is to examine the latest physiological and pharmacological findings highlighting the contribution of ligand gated ion channels including the inhibitory GABA_A and glycine receptors and excitatory nicotinic acetylcholine receptors and glutamate receptors in the sleep-wake cycle regulation. Overall, a better understanding of ligand gated ion channels in sleep will help determine if these highly druggable targets could facilitate a better night's sleep.

Dequalinium chloride is an antagonists of α7 nicotinic acetylcholine receptors

Dequalinium chloride has been used primarily as antiseptic compounds, but recently has been investigated for its effects on specific targets, including muscarinic acetylcholine receptors. Here we investigated dequalinium chloride as an antagonist to α7 nicotinic acetylcholine receptors. The pharmacological properties of dequalinium were established using cell lines stably co-transfected with the calcium-permeable human α7 nicotinic acetylcholine receptors and its chaperone NACHO, calcium dye fluorescent measurements or a calcium-sensitive protein reporter, and patch clamp recording of ionic currents. Using calcium dye fluorescence plate reader measurements, we find dequalinium chloride is an antagonist of α7 nicotinic acetylcholine receptors with an IC₅₀ of 672 nM in response to activation with 500 μM acetylcholine chloride and positive allosteric modulator PNU-120596. However, using a membrane-tethered GCAMP7s calcium reporter allowed detection of α7-mediated calcium flux in the absence of PNU-120596. Using this approach revealed an IC₅₀ of 157 nM for dequalinium on 300 μM acetylcholine-evoked currents. Using patch clamp recordings with 300 μM acetylcholine chloride and 10 μM PNU-120596, we find lower concentrations are sufficient to block ionic currents, with IC₅₀ of 120 nM for dequalinium chloride and 54 nM for the related UCL 1684 compound. In summary, we find that dequalinium chloride and UCL1684, which are generally used to block SK-type potassium channels, are also highly effective antagonists of α7 nicotinic acetylcholine receptors. This finding, in combination with previous studies of muscarinic acetylcholine receptors, clearly establishes dequalinium compounds within the class of general anti-cholinergic antagonists.

Scopolamine prevents aberrant mossy fiber sprouting and facilitates remission of epilepsy after brain injury

Prevention or modification of acquired epilepsy in patients at risk is an urgent, yet unmet, clinical need. Following acute brain insults, there is an increased risk of mesial temporal lobe epilepsy (mTLE), which is often associated with debilitating comorbidities and reduced life expectancy. The latent period between brain injury and the onset of epilepsy may offer a therapeutic window for interfering with epileptogenesis. The pilocarpine model of mTLE is widely used in the search for novel antiepileptogenic treatments. Recent biochemical studies indicated that cholinergic mechanisms play a role in the epileptogenic alterations induced by status epilepticus (SE) in this and other models of mTLE, which prompted us to evaluate whether treatment with the muscarinic antagonist scopolamine during the latent period after SE is capable of preventing or modifying epilepsy and associated behavioral and cognitive alterations in female Sprague-Dawley rats. First, in silico pharmacokinetic modeling was used to select a dosing protocol by which M-receptor inhibitory brain levels of scopolamine are maintained during prolonged treatment. This protocol was verified by drug analysis in vivo. Rats were then treated twice daily with scopolamine over 17 days after SE, followed by drug wash-out and behavioral and video/EEG monitoring up to ~6 months after SE. Compared to vehicle controls, rats that were treated with scopolamine during the latent period exhibited a significantly lower incidence of spontaneous recurrent seizures during periods of intermittent recording in the chronic phase of epilepsy, less behavioral excitability, less cognitive impairment, and significantly reduced aberrant mossy fiber sprouting in the hippocampus. The present data may indicate that scopolamine exerts antiepileptogenic/disease-modifying activity in the lithium-pilocarpine rat model, possibly involving increased remission of epilepsy as a new mechanism of disease-modification. For evaluating the rigor of the present data, we envision a study that more thoroughly addresses the gender bias and video-EEG recording limitations of the present study.

Nicotinic Receptors in Sleep-Related Hypermotor Epilepsy: Pathophysiology and Pharmacology

Sleep-related hypermotor epilepsy (SHE) is characterized by hyperkinetic focal seizures, mainly arising in the neocortex during non-rapid eye movements (NREM) sleep. The familial form is autosomal dominant SHE (ADSHE), which can be caused by mutations in genes encoding subunits of the neuronal nicotinic acetylcholine receptor (nAChR), Na⁺-gated K⁺ channels, as well as non-channel signaling proteins, such as components of the gap activity toward rags 1 (GATOR1) macromolecular complex. The causative genes may have different roles in developing and mature brains. Under this respect, nicotinic receptors are paradigmatic, as different pathophysiological roles are exerted by distinct nAChR subunits in adult and developing brains. The widest evidence concerns α4 and β2 subunits. These participate in heteromeric nAChRs that are major modulators of excitability in mature neocortical circuits as well as regulate postnatal synaptogenesis. However, growing evidence implicates mutant α2 subunits in ADSHE, which poses interpretive difficulties as very little is known about the function of α2-containing (α2*) nAChRs in the human brain. Planning rational therapy must consider that pharmacological treatment could have different effects on synaptic maturation and adult excitability. We discuss recent attempts towards precision medicine in the mature brain and possible approaches to target developmental stages. These issues have general relevance in epilepsy treatment, as the pathogenesis of genetic epilepsies is increasingly recognized to involve developmental alterations.

Control of neuronal excitability by GSK-3beta: Epilepsy and beyond

Glycogen synthase kinase 3beta (GSK-3β) is an enzyme with a variety of cellular functions in addition to the regulation of glycogen metabolism. In the central nervous system, different intracellular signaling pathways converge on GSK-3β through a cascade of phosphorylation events that ultimately control a broad range of neuronal functions in the development and adulthood. In mice, genetically removing or increasing GSK-3β cause distinct functional and structural neuronal phenotypes and consequently affect cognition. Precise control of GSK-3β activity is important for such processes as neuronal migration, development of neuronal morphology, synaptic plasticity, excitability, and gene expression. Altered GSK-3β activity contributes to aberrant plasticity within neuronal circuits leading to neurological, psychiatric disorders, and neurodegenerative diseases. Therapeutically targeting GSK-3β can restore the aberrant plasticity of neuronal networks at least in animal models of these diseases. Although the complete repertoire of GSK-3β neuronal substrates has not been defined, emerging evidence shows that different ion channels and their accessory proteins controlling excitability, neurotransmitter release, and synaptic transmission are regulated by GSK-3β, thereby supporting mechanisms of synaptic plasticity in cognition. Dysregulation of ion channel function by defective GSK-3β activity sustains abnormal excitability in the development of epilepsy and other GSK-3β-linked human diseases.

Differentiating the Pharmacodynamics and Toxicology of Macrolide and Ketolide Antibiotics

This is a review of the macrolide and ketolide field focusing on differentiating the pharmacodynamics and especially the toxicology of the macrolides and ketolides. We emphasize the diversity in pharmacodynamics and toxicity of the macrolides and ketolides, resulting from even small structural changes, which makes it important to consider the various different compounds separately, not necessarily as a class. The ketolide, telithromycin, was developed because of rising bacterial macrolide resistance but was withdrawn postapproval after visual disturbances, syncope, myasthenia gravis, and hepatotoxicity were noted. These diverse adverse effects could be attributed to inhibition of nicotinic acetylcholine receptors. Solithromycin, a later generation ketolide, was effective in treating bacterial pneumonia, but it was not approved by the U.S. Food and Drug Administration owing, in part, to its structural similarity to telithromycin. This Miniperspective describes that structurally similar macrolides/ketolides have clearly mechanistically distinct effects. Understanding these effects could help in developing and securing regulatory approval of a new macrolide/ketolide that is active against macrolide-resistant pathogenic bacteria.

L-3,3',5-triiodothyronine and pregnenolone sulfate inhibit Torpedo nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (nAChR) is an excitatory pentameric ligand-gated ion channel (pLGIC), homologous to the inhibitory γ-aminobutyric acid (GABA) type A receptor targeted by pharmaceuticals and endogenous sedatives. Activation of the GABA_A receptor by the neurosteroid allopregnanolone can be inhibited competitively by thyroid hormone (L-3,3’,5-triiodothyronine, or T3), but modulation of nAChR by T3 or neurosteroids has not been investigated. Here we show that allopregnanolone inhibits the nAChR from Torpedo californica at micromolar concentrations, as do T3 and the anionic neurosteroid pregnenolone sulfate (PS). We test for the role of protein and ligand charge in mediated receptor inhibition by varying pH in a narrow range around physiological pH. We find that both T3 and PS become less potent with increasing pH, with remarkably similar trends in IC₅₀ when T3 is neutral at pH < 7.3. After deprotonation of T3 (but no additional deprotonation of PS) at pH 7.3, T3 loses potency more slowly with increasing pH than PS. We interpret this result as indicating the negative charge is not required for inhibition but does increase activity. Finally, we show that both T3 and PS affect nAChR channel desensitization, which may implicate a binding site homologous to one that was recently indicated for accelerated desensitization of the GABA_A receptor by PS.

Nicotinic receptor dependent regulation of cough and other airway defensive reflexes

Nicotinic receptor activation in the airways evokes airway defensive reflexes including cough. These reflexes are the direct result of bronchopulmonary afferent nerve activation, which may occur directly, through activation of nicotinic receptors expressed on the terminals of airway sensory nerves, or indirectly, secondary to the end organ effects associated with autonomic nerve stimulation. The irritating effects of nicotine delivered topically to the airways are counterbalanced by an inhibitory effect of nicotinic receptor activation in the central nervous system. We present evidence that these nicotinic receptors are components of essential transducing and encoding mechanisms regulating airway defense.

Synthesis, Structure and Biological Activity of CIA and CIB, Two α-Conotoxins from the Predation-Evoked Venom of Conus catus

Cone snails produce a fast-acting and often paralyzing venom that is usually injected into their prey or predator through a hypodermic needle-like modified radula tooth. Many diverse compounds are found in their venom including small molecules, peptides and enzymes. However, peptidic toxins called conotoxins (10⁻40 residues and 2⁻4 disulfide bonds) largely dominate these cocktails. These disulfide rich toxins are very valuable pharmacological tools for investigating the function of ions channels, G-protein coupled receptors, transporters and enzymes. Here, we report on the synthesis, structure determination and biological activities of two α-conotoxins, CIA and CIB, found in the predatory venom of the piscivorous species Conus catus. CIA is a typical 3/5 α-conotoxin that blocks the rat muscle type nAChR with an IC₅₀ of 5.7 nM. Interestingly, CIA also inhibits the neuronal rat nAChR subtype α3β2 with an IC₅₀ of 2.06 μM. CIB is a 4/7 α-conotoxin that blocks rat neuronal nAChR subtypes, including α3β2 (IC₅₀ = 128.9 nM) and α7 (IC₅₀ = 1.51 μM). High resolution NMR structures revealed typical α-conotoxin folds for both peptides. We also investigated the in vivo effects of these toxins on fish, since both peptides were identified in the predatory venom of C. catus. Consistent with their pharmacology, CIA was highly paralytic to zebrafish (ED₅₀ = 110 μg/kg), whereas CIB did not affect the mobility of the fish. In conclusion, CIA likely participates in prey capture through muscle paralysis, while the putative ecological role of CIB remains to be elucidated.

α-Conotoxins to explore the molecular, physiological and pathophysiological functions of neuronal nicotinic acetylcholine receptors

The vast diversity of neuronal nicotinic acetylcholine subunits expressed in the central and peripheral nervous systems, as well as in non-neuronal tissues, constitutes a formidable challenge for researchers and clinicians to decipher the role of particular subtypes, including complex subunit associations, in physiological and pathophysiological functions. Many natural products target the nAChRs, but there is no richer source of nicotinic ligands than the venom of predatory gastropods known as cone snails. Indeed, every single species of cone snail was shown to produce at least one type of such α-conotoxins. These tiny peptides (10-25 amino acids), constrained by disulfide bridges, proved to be unvaluable tools to investigate the structure and function of nAChRs, some of them having also therapeutic potential. In this review, we provide a recent update on the pharmacology and subtype specificity of several major α-conotoxins.

Single versus combinatorial therapies in status epilepticus: Novel data from preclinical models

Drug-refractory status epilepticus (RSE) is a major medical emergency with a mortality of up to 40% and the risk of severe long-term consequences. The mechanisms involved in RSE are incompletely understood. Animal models are important in developing treatment strategies for more effective termination of SE and prevention of its long-term outcomes. The pilocarpine and lithium-pilocarpine rat models are widely used in this respect. In these models, resistance to diazepam and other antiseizure drugs (ASDs) develops during SE so that an SE that is longer than 30 min is difficult to suppress. Furthermore, because all ASDs used in SE treatment are much more rapidly eliminated by rodents than by humans, SE recurs several hours after ASD treatment. Long-term consequences include hippocampal damage, behavioral alterations, and epilepsy with spontaneous recurrent seizures. In this review, different rational polytherapies for SE, which are more effective than monotherapies, are discussed, including a novel polytherapy recently developed by our group. Based on data from diverse seizure models, we hypothesized that cholinergic mechanisms are involved in the mechanisms underlying ASD resistance of SE. We, therefore, developed an intravenous drug cocktail, consisting of diazepam, phenobarbital, and the anticholinergic scopolamine. This drug combination irreversibly terminated SE when administered 60, 90, or 120 min after SE onset. The efficacy of this cocktail in terminating SE was comparable with the previously reported efficacy of polytherapies with the glutamate receptor antagonist ketamine. Furthermore, when injected 60 min after SE onset, the scopolamine-containing cocktail prevented development of epilepsy and hippocampal neurodegeneration, which was not observed with high doses of diazepam or a combination of phenobarbital and diazepam. Our data add to the existing preclinical evidence that rational polytherapy can be more effective than monotherapy in the treatment of SE and that combinatorial therapy may offer a clinically useful option for the treatment of RSE. This article is part of a Special Issue entitled "Status Epilepticus".

Ionotropic GABA and Glutamate Receptor Mutations and Human Neurologic Diseases

The advent of whole exome/genome sequencing and the technology-driven reduction in the cost of next-generation sequencing as well as the introduction of diagnostic-targeted sequencing chips have resulted in an unprecedented volume of data directly linking patient genomic variability to disorders of the brain. This information has the potential to transform our understanding of neurologic disorders by improving diagnoses, illuminating the molecular heterogeneity underlying diseases, and identifying new targets for therapeutic treatment. There is a strong history of mutations in GABA receptor genes being involved in neurologic diseases, particularly the epilepsies. In addition, a substantial number of variants and mutations have been found in GABA receptor genes in patients with autism, schizophrenia, and addiction, suggesting potential links between the GABA receptors and these conditions. A new and unexpected outcome from sequencing efforts has been the surprising number of mutations found in glutamate receptor subunits, with the GRIN2A gene encoding the GluN2A N-methyl-d-aspartate receptor subunit being most often affected. These mutations are associated with multiple neurologic conditions, for which seizure disorders comprise the largest group. The GluN2A subunit appears to be a locus for epilepsy, which holds important therapeutic implications. Virtually all α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor mutations, most of which occur within GRIA3, are from patients with intellectual disabilities, suggesting a link to this condition. Similarly, the most common phenotype for kainate receptor variants is intellectual disability. Herein, we summarize the current understanding of disease-associated mutations in ionotropic GABA and glutamate receptor families, and discuss implications regarding the identification of human mutations and treatment of neurologic diseases.

Spontaneous epileptic seizures in transgenic rats harboring a human ADNFLE missense mutation in the β2-subunit of the nicotinic acetylcholine receptor

We generated a transgenic rat strain with a missense mutation in V286L (V286L-TG), in the gene encoding the neuronal nicotinic acetylcholine receptor β2 subunit (CHRNB2) found in patients with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). To confirm that V286L-TG rats exhibit seizures similar to those observed in humans, gene expression patterns and behavioral phenotypes were analyzed. In situ hybridization using a V286L Chrnb2-selective probe indicated that the transgene was expressed at higher levels in the cortex, hippocampus, and cerebellum of V286L-TG than wild-type littermates (non-TG). Spontaneous epileptic seizures with ictal discharges in electroencephalograms were detected in 45% of V286L-TG rats and the frequency of seizures was 0.73 times a week. This seizure type is similar to "paroxysmal arousals" that are observed in human ADNFLE. V286L-TG rats displayed nicotine-induced abnormal motor activity including seizures in comparison to non-TGs. Response time following nicotine administration occurred faster in V286L-TG than in non-TG rats. V286L-TG rats demonstrated spontaneous epileptic seizures, which are similar to human ADNFLE, and also showed a higher sensitivity to nicotine administration. Thus, the V286L-TG rat model could be a valuable tool for developing novel mechanism-driven treatment strategies for epilepsy and provide a better understanding of ADNFLE.

Effective termination of status epilepticus by rational polypharmacy in the lithium-pilocarpine model in rats: Window of opportunity to prevent epilepsy and prediction of epilepsy by biomarkers

The pilocarpine rat model, in which status epilepticus (SE) leads to epilepsy with spontaneous recurrent seizures (SRS), is widely used to study the mechanisms of epileptogenesis and develop strategies for epilepsy prevention. SE is commonly interrupted after 30-90min by high-dose diazepam or other anticonvulsants to reduce mortality. It is widely believed that SE duration of 30-60min is sufficient to induce hippocampal damage and epilepsy. However, resistance to diazepam develops during SE, so that an SE that is longer than 30min is difficult to terminate, and SE typically recurs several hours after diazepam, thus forming a bias for studies on epileptogenesis or antiepileptogenesis. We developed a drug cocktail, consisting of diazepam, phenobarbital, and scopolamine that allows complete and persistent SE termination in the lithium-pilocarpine model. A number of novel findings were obtained with this cocktail. (a) In contrast to previous reports with incomplete SE suppression, a SE of 60min duration did not induce epilepsy, whereas epilepsy with SRS developed after 90 or 120min SE; (b) by comparing groups of rats with 60 and 90min of SE, development of epilepsy could be predicted by behavioral hyperexcitability and decrease in seizure threshold, indicating that these read-outs are suited as biomarkers of epileptogenesis; (c) CA1 damage was prevented by the cocktail, but rats exhibited cell loss in the dentate hilus, which was related to development of epilepsy. These data demonstrate that the duration of SE needed for induction of epileptogenesis in this model is longer than previously thought.

Discovery of a new subclass of α-conotoxins in the venom of Conus australis

Cone snails (Conus sp.) are poisonous animals that can be found in all oceans where they developed a venomous strategy to prey or to defend. The venom of these species contains an undeniable source of unique and potent pharmacologically active compounds. Their peptide compounds, called conotoxins, are not only interesting for the development of new pharmaceutical ligands, but they are also useful for studying their broad spectrum of targets. One conotoxin family in particular, the α-conotoxins, acts on nicotinic acetylcholine receptors (nAChRs) which dysfunctions play important roles in pathologies such as epilepsy, myasthenic syndromes, schizophrenia, Parkinson's disease and Alzheimer's disease. Here we define a new subclass of the α-conotoxin family. We purified the venom of a yet unexplored cone snail species, i.e. Conus australis, and we isolated a 16-amino acid peptide named α-conotoxin AusIA. The peptide has the typical α-conotoxin CC-Xm-C-Xn-C framework, but both loops (m/n) contain 5 amino acids, which has never been described before. Using conventional electrophysiology we investigated the response of synthetically made globular (I-III, II-IV) and ribbon (I-IV, II-III) AusIA to different nicotinic acetylcholine receptors. The α7 nAChR was the only receptor found to be blocked with a similar potency by both peptide-configurations. This suggests that both α5/5 conotoxin isomers might be present in the venom gland of C. australis. NMR spectroscopy showed that no secondary structures define the peptides' three-dimensional topology. Moreover, the ribbon configuration, which is generally considered to be non-native, is more stable than the globular isomer. Accordingly, our findings show relevancy concerning the α-conotoxin classification which might be helpful in the design of novel therapeutic compounds.

Purification and functional characterization of a truncated human α4β2 nicotinic acetylcholine receptor

Nicotinic acetylcholine receptors (nAChR) are abundant in the brain and are essential in cognitive function, learning and memory. Previous efforts on α4β2 nAChR had been focused on functional and pharmacological characterization, where high expression yield is not essential. For structural studies though, large amounts of pure protein is important but heterologous overexpression of membrane proteins can be a burdensome task, especially if high amounts are required. In the current study, a truncated mutant of the human α4β2 nAChR was designed in order to improve expression and solubility and to obtain material suitable for high resolution structural studies. We showed that the wild type α4β2 nAChR presented low expression and solubilization yield both of which were improved with the truncated construct. The truncated nAChR showed similar binding profile to the wild type, was purified by a two-step chromatography and isolated in high purity and adequate quantity.

Conotoxins targeting nicotinic acetylcholine receptors: an overview

Marine snails of the genus Conus are a large family of predatory gastropods with an unparalleled molecular diversity of pharmacologically active compounds in their venom. Cone snail venom comprises of a rich and diverse cocktail of peptide toxins which act on a wide variety of ion channels such as voltage-gated sodium- (Na_V), potassium- (K_V), and calcium- (Ca_V) channels as well as nicotinic acetylcholine receptors (nAChRs) which are classified as ligand-gated ion channels. The mode of action of several conotoxins has been the subject of investigation, while for many others this remains unknown. This review aims to give an overview of the knowledge we have today on the molecular pharmacology of conotoxins specifically interacting with nAChRs along with the structure–function relationship data.

Structure-function elucidation of a new α-conotoxin, Lo1a, from Conus longurionis

α-Conotoxins are peptide toxins found in the venom of marine cone snails and potent antagonists of various subtypes of nicotinic acetylcholine receptors (nAChRs). nAChRs are cholinergic receptors forming ligand-gated ion channels in the plasma membranes of certain neurons and the neuromuscular junction. Because nAChRs have an important role in regulating transmitter release, cell excitability, and neuronal integration, nAChR dysfunctions have been implicated in a variety of severe pathologies such as epilepsy, myasthenic syndromes, schizophrenia, Parkinson disease, and Alzheimer disease. To expand the knowledge concerning cone snail toxins, we examined the venom of Conus longurionis. We isolated an 18-amino acid peptide named α-conotoxin Lo1a, which is active on nAChRs. To the best of our knowledge, this is the first characterization of a conotoxin from this species. The peptide was characterized by electrophysiological screening against several types of cloned nAChRs expressed in Xenopus laevis oocytes. The three-dimensional solution structure of the α-conotoxin Lo1a was determined by NMR spectroscopy. Lo1a, a member of the α4/7 family, blocks the response to acetylcholine in oocytes expressing α7 nAChRs with an IC50 of 3.24 ± 0.7 μM. Furthermore, Lo1a shows a high selectivity for neuronal versus muscle subtype nAChRs. Because Lo1a has an unusual C terminus, we designed two mutants, Lo1a-ΔD and Lo1a-RRR, to investigate the influence of the C-terminal residue. Lo1a-ΔD has a C-terminal Asp deletion, whereas in Lo1a-RRR, a triple-Arg tail replaces the Asp. They blocked the neuronal nAChR α7 with a lower IC50 value, but remarkably, both adopted affinity for the muscle subtype α1β1δε.

The role of dopamine signaling in epileptogenesis

Clinical and experimental studies implicate most neuromodulatory systems in epileptogenesis. The dopaminergic system has a seizure-modulating effect that crucially depends on the different subtypes of dopamine (DA) receptors involved and the brain regions in which they are activated. Specifically, DA plays a major role in the control of seizures arising in the limbic system. Studies performed in a wide variety of animal models contributed to illustrate the opposite actions of D1-like and D2-like receptor signaling in limbic epileptogenesis. Indeed, signaling from D1-like receptors is generally pro-epileptogenic, whereas D2-like receptor signaling exerts an anti-epileptogenic effect. However, this view might appear quite simplistic as the complex neuromodulatory action of DA in the control of epileptogenesis likely requires a physiological balance in the activation of circuits modulated by these two major DA receptor subtypes, which determines the response to seizure-promoting stimuli. Here we will review recent evidences on the identification of molecules activated by DA transduction pathways in the generation and spread of seizures in the limbic system. We will discuss the intracellular signaling pathways triggered by activation of different DA receptors in relation to their role in limbic epileptogenesis, which lead to the activation of neuronal death/survival cascades. A deep understanding of the signaling pathways involved in epileptogenesis is crucial for the identification of novel targets for the treatment of epilepsy.

Are vesicular neurotransmitter transporters potential treatment targets for temporal lobe epilepsy?

The vesicular neurotransmitter transporters (VNTs) are small proteins responsible for packing synaptic vesicles with neurotransmitters thereby determining the amount of neurotransmitter released per vesicle through fusion in both neurons and glial cells. Each transporter subtype was classically seen as a specific neuronal marker of the respective nerve cells containing that particular neurotransmitter or structurally related neurotransmitters. More recently, however, it has become apparent that common neurotransmitters can also act as co-transmitters, adding complexity to neurotransmitter release and suggesting intriguing roles for VNTs therein. We will first describe the current knowledge on vesicular glutamate transporters (VGLUT1/2/3), the vesicular excitatory amino acid transporter (VEAT), the vesicular nucleotide transporter (VNUT), vesicular monoamine transporters (VMAT1/2), the vesicular acetylcholine transporter (VAChT) and the vesicular γ-aminobutyric acid (GABA) transporter (VGAT) in the brain. We will focus on evidence regarding transgenic mice with disruptions in VNTs in different models of seizures and epilepsy. We will also describe the known alterations and reorganizations in the expression levels of these VNTs in rodent models for temporal lobe epilepsy (TLE) and in human tissue resected for epilepsy surgery. Finally, we will discuss perspectives on opportunities and challenges for VNTs as targets for possible future epilepsy therapies.

A common susceptibility factor of both autism and epilepsy: functional deficiency of GABA A receptors

Autism and epilepsy are common childhood neurological disorders with a great heterogeneity of clinical phenotypes as well as risk factors. There is a high co-morbidity of autism and epilepsy. The neuropathology of autism and epilepsy has similar histology implicating the processes of neurogenesis, neural migration, programmed cell death, and neurite outgrowth. Genetic advances have identified multiple molecules that participate in neural development, brain network connectivity, and synaptic function which are involved in the pathogenesis of autism and epilepsy. Mutations in GABA(A) receptor subunit have been frequently associated with epilepsy, autism, and other neuropsychiatric disorders. In this paper, we address the hypothesis that functional deficiency of GABAergic signaling is a potential common molecular mechanism underpinning the co-morbidity of autism and epilepsy.

Genetic matters: thirty years of progress using mouse models in nicotinic research

This research update summarizes thirty years of studies on genetic influences on responses to the acute or chronic administration of nicotine. Early studies established that various inbred mice are differentially sensitive to the effects of the drug. Classical genetic analyses confirmed that nicotine effects on locomotion, body temperature and seizures are heritable. A significant inverse correlation between the locomotor and hypothermic effects and the density of nicotine binding sites suggested that differential expression α4β2-neuronal nicotinic acetylcholine receptor (nAChR) mediated some of this genetic variability. Subsequent studies with α4 and β2 nAChR null (decreased sensitivity) and gain of function mutants (increased sensitivity) supports the role of the α4β2*nAChR subtype. However, null mutant mice still respond to nicotine, indicating that other nAChR subtypes also mediate these responses. Mice differing in initial sensitivity to nicotine also differ in tolerance development following chronic treatment: those mice that are initially more sensitive to nicotine develop tolerance at lower treatment doses than less sensitive mice, indicating that tolerance is an adaptive response to the effects of nicotine. In contrast, the sensitivity of mice to pre-pulse inhibition of acoustic startle response is correlated with the expression of α7-nAChR. While genetic variability in nAChR expression and function is an important factor contributing to differences in response to nicotine, the observations that altered activity of opioid, glutamate, and cannabinoid receptors among others also change nicotine sensitivity reinforces the proposal that the genetics of nicotine response is more complex than differences in nAChRs.

Nocturnal frontal lobe epilepsy and the acetylcholine receptor

BACKGROUND

Nocturnal frontal lobe epilepsy (NFLE) is an idiopathic partial epilepsy characterized by a wide spectrum of stereotyped motor manifestations, mostly occurring during non rapid eye movements sleep. NFLE is underdiagnosed since semiological similarities make it difficult to distinguish NFLE from parasomnias. In 1994, authors reported families with NFLE inherited as an autosomal dominant trait and they introduced the term of autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). A family history of possible NFLE is found in about 25% of cases. The genetic bases of the disease have been detected in a minority of cases. Mutations causing a gain of function of the neuronal nicotinic acetylcholine receptors were reported in 3 different subunits.

REVIEW SUMMARY

This review discusses the clinical aspects of NFLE and the diagnostic procedures. Furthermore, the genetic aspects are outlined. The main differentiating features characterizing NFLE are: (a) several attacks per night at any time during the night; (b) brief duration of the attacks; (c) stereotyped motor pattern. Nocturnal video-polysomnography is crucial for the diagnosis. Neurological examination in NFLE/ADNFLE is normal. About 30% of NFLE cases are resistant to antiepileptic drugs. Concerning the genetics, putative susceptibility nucleotide variations affecting the promoter of the CRH gene and altering the corticotrophin-releasing hormone levels have been reported in some NFLE patients.

CONCLUSIONS

Distinguishing NFLE seizures from paroxysmal nonepileptic sleep disorders is often difficult and sometimes impossible on clinical grounds alone. Nocturnal video-polysomnography is mandatory. Further genetic studies could help the diagnosis and treatment in NFLE patients.

Alpha7 neuronal nicotinic receptors as a drug target in schizophrenia

INTRODUCTION

Schizophrenia is a profoundly debilitating disease that represents not only an individual, but a societal problem. Once characterized solely by the hyperactivity of the dopaminergic system, therapies directed to dampen dopaminergic neurotransmission were developed. However, these drugs do not address the significant impairments in cognition and the negative symptoms of the disease, and it is now apparent that disequilibrium of many neurotransmitter systems is involved. Despite enormous efforts, minimal progress has been made toward the development of safer, more effective therapies to date.

AREAS COVERED

The high preponderance of smoking in schizophrenics suggests that nicotine may provide symptomatic improvement, which has led to investigation for selective molecules targeted to individual nicotinic receptor (nAChR) subtypes. Of special interest is activation of the homomeric α7nAChR, which is widely distributed in the brain and has been implicated in the pathophysiology of schizophrenia through numerous approaches.

EXPERT OPINION

Preclinical and clinical data suggest that neuronal α7nAChRs play an important role in cognitive functions. Moreover, some, but not all, early clinical trials conducted with α7nAChR agonists show cognitive benefits in schizophrenics. These encouraging results suggest that development of compounds targeting α7nAChRs will represent a valuable tool to mitigate symptoms associated with schizophrenia, and open new strategies for better pharmacological treatment of these patients.

Genome-wide association analysis of genetic generalized epilepsies implicates susceptibility loci at 1q43, 2p16.1, 2q22.3 and 17q21.32

Genetic generalized epilepsies (GGEs) have a lifetime prevalence of 0.3% and account for 20-30% of all epilepsies. Despite their high heritability of 80%, the genetic factors predisposing to GGEs remain elusive. To identify susceptibility variants shared across common GGE syndromes, we carried out a two-stage genome-wide association study (GWAS) including 3020 patients with GGEs and 3954 controls of European ancestry. To dissect out syndrome-related variants, we also explored two distinct GGE subgroups comprising 1434 patients with genetic absence epilepsies (GAEs) and 1134 patients with juvenile myoclonic epilepsy (JME). Joint Stage-1 and 2 analyses revealed genome-wide significant associations for GGEs at 2p16.1 (rs13026414, P(meta) = 2.5 × 10(-9), OR[T] = 0.81) and 17q21.32 (rs72823592, P(meta) = 9.3 × 10(-9), OR[A] = 0.77). The search for syndrome-related susceptibility alleles identified significant associations for GAEs at 2q22.3 (rs10496964, P(meta) = 9.1 × 10(-9), OR[T] = 0.68) and at 1q43 for JME (rs12059546, P(meta) = 4.1 × 10(-8), OR[G] = 1.42). Suggestive evidence for an association with GGEs was found in the region 2q24.3 (rs11890028, P(meta) = 4.0 × 10(-6)) nearby the SCN1A gene, which is currently the gene with the largest number of known epilepsy-related mutations. The associated regions harbor high-ranking candidate genes: CHRM3 at 1q43, VRK2 at 2p16.1, ZEB2 at 2q22.3, SCN1A at 2q24.3 and PNPO at 17q21.32. Further replication efforts are necessary to elucidate whether these positional candidate genes contribute to the heritability of the common GGE syndromes.

Reversal of impaired hippocampal long-term potentiation and contextual fear memory deficits in Angelman syndrome model mice by ErbB inhibitors

BACKGROUND

Angelman syndrome (AS) is a human neuropsychiatric disorder associated with autism, mental retardation, motor abnormalities, and epilepsy. In most cases, AS is caused by the deletion of the maternal copy of UBE3A gene, which encodes the enzyme ubiquitin ligase E3A, also termed E6-AP. A mouse model of AS has been generated and these mice exhibit many of the observed neurological alterations in humans. Because of clinical and neuroanatomical similarities between AS and schizophrenia, we examined AS model mice for alterations in the neuregulin-ErbB4 pathway, which has been implicated in the pathophysiology of schizophrenia. We focused our studies on the hippocampus, one of the major brain loci impaired in AS mice.

METHODS

We determined the expression of neuregulin 1 and ErbB4 receptors in AS mice and wild-type littermates (ages 10-16 weeks) and studied the effects of ErbB inhibition on long-term potentiation in hippocampal area cornu ammonis 1 and on hippocampus-dependent contextual fear memory.

RESULTS

We observed enhanced neuregulin-ErbB4 signaling in the hippocampus of AS model mice and found that ErbB inhibitors could reverse deficits in long-term potentiation, a cellular substrate for learning and memory. In addition, we found that an ErbB inhibitor enhanced long-term contextual fear memory in AS model mice.

CONCLUSIONS

Our findings suggest that neuregulin-ErbB4 signaling is involved in synaptic plasticity and memory impairments in AS model mice, suggesting that ErbB inhibitors have therapeutic potential for the treatment of AS.

Vulnerability to paroxysmal oscillations in delayed neural networks: a basis for nocturnal frontal lobe epilepsy?

Resonance can occur in bistable dynamical systems due to the interplay between noise and delay (τ) in the absence of a periodic input. We investigate resonance in a two-neuron model with mutual time-delayed inhibitory feedback. For appropriate choices of the parameters and inputs three fixed-point attractors co-exist: two are stable and one is unstable. In the absence of noise, delay-induced transient oscillations (referred to herein as DITOs) arise whenever the initial function is tuned sufficiently close to the unstable fixed-point. In the presence of noisy perturbations, DITOs arise spontaneously. Since the correlation time for the stationary dynamics is ∼τ, we approximated a higher order Markov process by a three-state Markov chain model by rescaling time as t → 2sτ, identifying the states based on whether the sub-intervals were completely confined to one basin of attraction (the two stable attractors) or straddled the separatrix, and then determining the transition probability matrix empirically. The resultant Markov chain model captured the switching behaviors including the statistical properties of the DITOs. Our observations indicate that time-delayed and noisy bistable dynamical systems are prone to generate DITOs as switches between the two attractors occur. Bistable systems arise transiently in situations when one attractor is gradually replaced by another. This may explain, for example, why seizures in certain epileptic syndromes tend to occur as sleep stages change.

Pharmacogenetics of new analgesics

Patient phenotypes in pharmacological pain treatment varies between individuals, which could be partly assigned to their genotypes regarding the targets of classical analgesics (OPRM1, PTGS2) or associated signalling pathways (KCNJ6). Translational and genetic research have identified new targets, for which new analgesics are being developed. This addresses voltage-gated sodium, calcium and potassium channels, for which SCN9A, CACNA1B, KCNQ2 and KCNQ3, respectively, are primary gene candidates because they code for the subunits of the respective channels targeted by analgesics currently in clinical development. Mutations in voltage gated transient receptor potential (TRPV) channels are known from genetic pain research and may modulate the effects of analgesics under development targeting TRPV1 or TRPV3. To this add ligand-gated ion channels including nicotinic acetylcholine receptors, ionotropic glutamate-gated receptors and ATP-gated purinergic P2X receptors with most important subunits coded by CHRNA4, GRIN2B and P2RX7. Among G protein coupled receptors, δ-opioid receptors (coded by OPRD1), cannabinoid receptors (CNR1 and CNR2), metabotropic glutamate receptors (mGluR5 coded by GRM5), bradykinin B(1) (BDKRB1) and 5-HT(1A) (HTR1A) receptors are targeted by new analgesic substances. Finally, nerve growth factor (NGFB), its tyrosine kinase receptor (NTRK1) and the fatty acid amide hydrolase (FAAH) have become targets of interest. For most of these genes, functional variants have been associated with neuro-psychiatric disorders and not yet with analgesia. However, research on the genetic modulation of pain has already identified variants in these genes, relative to pain, which may facilitate the pharmacogenetic assessments of new analgesics. The increased number of candidate pharmacogenetic modulators of analgesic actions may open opportunities for the broader clinical implementation of genotyping information.

Identification of a negative allosteric site on human α4β2 and α3β4 neuronal nicotinic acetylcholine receptors

Acetylcholine-based neurotransmission is regulated by cationic, ligand-gated ion channels called nicotinic acetylcholine receptors (nAChRs). These receptors have been linked to numerous neurological diseases and disorders such as Alzheimer's disease, Parkinson's disease, and nicotine addiction. Recently, a class of compounds has been discovered that antagonize nAChR function in an allosteric fashion. Models of human α4β2 and α3β4 nicotinic acetylcholine receptor (nAChR) extracellular domains have been developed to computationally explore the binding of these compounds, including the dynamics and free energy changes associated with ligand binding. Through a blind docking study to multiple receptor conformations, the models were used to determine a putative binding mode for the negative allosteric modulators. This mode, in close proximity to the agonist binding site, is presented in addition to a hypothetical mode of antagonism that involves obstruction of C loop closure. Molecular dynamics simulations and MM-PBSA free energy of binding calculations were used as computational validation of the predicted binding mode, while functional assays on wild-type and mutated receptors provided experimental support. Based on the proposed binding mode, two residues on the β2 subunit were independently mutated to the corresponding residues found on the β4 subunit. The T58K mutation resulted in an eight-fold decrease in the potency of KAB-18, a compound that exhibits preferential antagonism for human α4β2 over α3β4 nAChRs, while the F118L mutation resulted in a loss of inhibitory activity for KAB-18 at concentrations up to 100 µM. These results demonstrate the selectivity of KAB-18 for human α4β2 nAChRs and validate the methods used for identifying the nAChR modulator binding site. Exploitation of this site may lead to the development of more potent and subtype-selective nAChR antagonists which may be used in the treatment of a number of neurological diseases and disorders.

Naturally-expressed nicotinic acetylcholine receptor subtypes

Nicotinic acetylcholine receptors (nAChRs) warrant attention, as they play many critical roles in brain and body function and have been implicated in a number of neurological and psychiatric disorders, including nicotine dependence. nAChRs are composed as diverse subtypes containing specific combinations of genetically-distinct subunits and that have different functional properties, distributions, and pharmacological profiles. There had been confidence that the rules that define ranges of assembly partners for specific subunits were well-established, especially for the more prominent nAChR subtypes. However, we review here some newer findings indicating that nAChRs having largely the same, major subunits exist as isoforms with unexpectedly different properties. Moreover, we also summarize our own studies indicating that novel nAChR subtypes exist and/or have distributions not heretofore described. Importantly, the nAChRs that exist as new isoforms or subtypes or have interesting distributions require alteration in thinking about their roles in health and disease.

Altered intrinsic properties of neuronal subtypes in malformed epileptogenic cortex

Neuronal intrinsic properties control action potential firing rates and serve to define particular neuronal subtypes. Changes in intrinsic properties have previously been shown to contribute to hyperexcitability in a number of epilepsy models. Here we examined whether a developmental insult producing the cortical malformation of microgyria altered the identity or firing properties of layer V pyramidal neurons and two interneuron subtypes. Trains of action potentials were elicited with a series of current injection steps during whole cell patch clamp recordings. Cells in malformed cortex identified as having an apical dendrite had firing patterns similar to control pyramidal neurons. The duration of the second action potential in the train was increased in paramicrogyral (PMG) pyramidal cells, suggesting that these cells may be in an immature state, as was previously found for layer II/III pyramidal neurons. Based on stereotypical firing patterns and other intrinsic properties, fast-spiking (FS) and low threshold-spiking (LTS) interneuron subpopulations were clearly identified in both control and malformed cortex. Most intrinsic properties measured in malformed cortex were unchanged, suggesting that subtype identity is maintained. However, LTS interneurons in lesioned cortex had increased maximum firing frequency, decreased initial afterhyperpolarization duration, and increased total adaptation ratio compared to control LTS cells. FS interneurons demonstrated decreased maximum firing frequencies in malformed cortex compared to control FS cells. These changes may increase the efficacy of LTS while decreasing the effectiveness of FS interneurons. These data indicate that differential alterations of individual neuronal subpopulations may endow them with specific characteristics that promote epileptogenesis.

Effect of carbamazepine and oxcarbazepine on wild-type and mutant neuronal nicotinic acetylcholine receptors linked to nocturnal frontal lobe epilepsy

Carbamazepine (5H-dibenz[b,f]azepine-5-carboxamide) and oxcarbazepine (10,11-dihydro-10-oxo-5H-dibenz[b,f]azepine-5-carboxamide) are widely used for the treatment of partial epilepsy. Recent work indicates that these drugs, in addition to targeting voltage-gated Na(+) channels, can modulate ligand-gated channels. These compounds appear to be particularly effective for treatment of nocturnal frontal lobe epilepsy, which can be caused by mutant neuronal nicotinic receptors. We compared the effects of carbamazepine and oxcarbazepine on heteromeric nicotinic receptors to better understand the underlying mechanism of the effect of these drugs in epileptic patients. Receptors were expressed in cell lines and studied by patch-clamp methods at -60 mV. For alpha2beta4 receptors activated with 100 microM nicotine, IC(50) for carbamazepine was 49 microM. Receptors in which alpha2 was substituted with alpha2-I279 N, linked to autosomal dominant nocturnal frontal lobe epilepsy, had an IC(50) of 21 microM. For oxcarbazepine, the IC(50) was larger than 500 microM for wild-type receptors and approximately 100 microM for mutant receptors. A similar inhibition was observed in the presence of 10 microM nicotine, indicating a non-competitive mechanism. The monohydroxy derivative (MHD) of oxcarbazepine, clinically the most relevant compound, was tested on both alpha2beta4 and alpha4beta2 receptors, to obtain a broader view of its possible physiological effects. At the typical concentration present in blood (100 microM), MHD produced an approximate 40% channel block on alpha4beta2, but no significant effect on alpha2beta4 receptors. Oxcarbazepine and MHD retarded the channel deactivation, suggesting that these compounds produce open channel block. These results may explain the particular efficacy of these drugs in nocturnal frontal lobe epilepsy.